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NEW YORK (Reuters) - U.S. stocks fell in a thinly traded session on Friday as the S&P 500 index recorded its steepest decline since May 2012 and investors avoided making large bets before a long weekend with the situation about Syria still uncertain.

Afternoon trading was volatile, with indexes swinging between break-even levels and solid losses as U.S. Secretary of State John Kerry said in televised remarks that Syria's government used poison gas against civilians and made the case for a limited military response.

President Barack Obama said later he has not made a final decision on a response to Syria.

"People are uneasy not knowing what's going on," said John Carey, portfolio manager at Pioneer Investment Management in Boston. "With that uncertainty and going into the Labor Day holiday, we're seeing people step back."

The S&P 500 fell 3.1 percent in August and lost 1.8 percent for the week in a third decline in the past four weeks.

The CBOE Volatility index .VIX rose 2.2 percent, bringing its weekly rise to 22 percent. Trading was light ahead of Monday's market holiday for Labor Day. About 3.99 billion shares changing hands on the New York Stock Exchange, the Nasdaq and NYSE MKT, below the daily average so far this year of about 6.31 billion shares.

"I tend to view the weakness as a buying opportunity, barring some global crisis," said Carey, who helps oversee about $200 billion in assets. "Syria isn't the crisis in and of itself, but if we do take military action, there could be repercussions." Military action "is always very risky."

The Dow Jones industrial average .DJI was down 33.26 points, or 0.22 percent, at 14,807.69. The Standard & Poor's 500 Index .SPX fell 5.40 points, or 0.33 percent, at 1,632.77. The Nasdaq Composite Index .IXIC was down 30.44 points, or 0.84 percent, at 3,589.87.

The Nasdaq fell 1.9 percent for the week while the Dow slid 1.3 percent in its fourth straight weekly loss. For the month, the Dow fell 4.4 percent and the Nasdaq lost 1 percent. Only one of the 30 Dow components, Microsoft Corp (MSFT.O), ended higher for the month.

Almost 70 percent of stocks traded on the New York Stock Exchange closed lower while 73 percent of Nasdaq-listed shares ended in negative territory.

Weak data on individual spending and tame inflation painted a picture of a soft economy, keeping investors guessing when the Federal Reserve might start to cut back on stimulus measures.

Consumer spending rose only 0.1 percent and inflation was tame in July, with a price index for consumer spending up 0.1 percent.

Other data showed the pace of business activity in the U.S. Midwest increased in August, as the Institute for Supply Management-Chicago business barometer rose to 53.0 from 52.3 in July, matching economists' expectations.

The Thomson Reuters/University of Michigan's final reading on the overall index on consumer sentiment slipped to 82.1 in August from 85.1 in July, but managed to top economists' expectations for a final read of 80.5.

Salesforce.com Inc (CRM.N) jumped 12.6 percent to $49.13 and was the best performer in the S&P 500 after the company raised its fiscal 2014 sales outlook after reporting better-than-expected revenue and earnings.

Apache Corp (APA.N) climbed 9 percent to $85.68 after the oil and gas producer said it was selling a 33 percent stake in its Egypt oil and gas business for $3.1 billion to state-owned Chinese oil giant Sinopec Group.

Omnivision Technologies Inc (OVTI.O) tumbled 16 percent to $15.45 after the chipmaker forecast current-quarter adjusted profit largely below expectations as rising competition and a slowdown of U.S. smartphone sales led to an inventory pile-up.